Currently, the steel used in the manufacture of roller bearing components with the best possible hardenability is believed to be the steel according to DIN 100CrMo7 4, containing 0.92-1.05% C, 0.25-0.4% Si, 0.6-0.8% Mn, 1.65-1.95% Cr, and 0.4-0.5% Mo.
In the manufacture of thick-walled components, such as large roller bearing rings having a wall thickness of 200 mm and up, it is very difficult if not impossible to obtain through hardening of the component. Without proper through hardening, the core will be unstable resulting in large deformations of the component during the hardening and also a reduction of the strength of the component. These large deformations necessitate manufacture of the component in dimensions which allow grinding of the component into the desired measures and straightness. Grinding of one ring may take up to 2 weeks, which adds considerably to the costs.
Generally, in order to improve hardenability, carbide formers as Cr and Mo are added. However, because of the high carbon content of bearing steels, additional amounts of Cr and Mo can cause excessive formation of easily fusible carbides which segregate and are enriched in the core, leading to porosity. In order to be able to produce dimensionally stable, through hardenable thick-walled components, there is a long felt, yet unfulfilled need for an improved bearing steel.